(57) Disclosed are an adsorbent for CO, comprising a composite comprised of a porous inorganic carrier and, carried thereon, a binary complex of a nitrogen-containing compound and a copper(I) halide, the nitrogen-containing compound being at least one member selected from at least one pyridine compound and a diamine represented by R¹R²N(CH₂)_n-NR³R⁴, wherein n is 2 or 3 and each of R¹, R², R³ and R⁴ independently represents a hydrogen atom or a C₁ - C₄ alkyl group, with the proviso that when n is 2, each of at least two of R¹, R², R³ and R⁴ represents a C₁ - C₄ alkyl group atoms and that when n is 3, at least one of R¹, R², R³ and R⁴ represents a C₁ - C₄ alkyl group, and a method for separating carbon monoxide by adsorption using the adsorbent. Also disclosed are a method for increasing the concentration of CO in an atmosphere by desorbing and releasing the CO from the adsorbent having CO adsorbed thereon and a method for regenerating an adsorbent by treating the deteriorated adsorbent (deteriorated by the contact with an oxygen-containing gas) with a reducing agent. The adsorbent is advantageous in that the adsorbent can adsorb CO with high selectivity and has high adsorptive and desorptive ability for CO, that the adsorbent can be produced using a conventional carrier and without using hydrochloric acid, with economical advantages, and that since the adsorbent is a solid, it can be handled with ease.